The heart pumps blood throughout the body in response to natural electrical stimuli which occur in the atrium and ventricle of the heart. If the natural electrical stimuli are disrupted or lost, the heart will not provide the circulation needed to sustain life. Implanted electrical devices which stimulate the heart have been used to correct various conditions. A pacemaker can be used to correct bradycardia, the condition wherein the heart beats too slowly. The heart may also beat too quickly (an arrhythmia or tachycardia) or it may quiver erratically (fibrillation). Electrical devices, including implantable defibrillators, have been designed to respond to such conditions and provide therapeutic electrical stimulation to the heart.
In using these devices, it is frequently desirable for a physician to be able to study the waveforms being produced in the heart, either naturally or in response to stimulation. In general, the implanted device senses the electrical condition of the heart through one or more leads implanted in or near the heart tissue. The sensed phenomenon can be transmitted directly from the implanted device to an external monitor for study by a physician, or the waveforms may be stored in some fashion and transmitted later. In either situation, it has been recognized that storage or transmission of direct analog data is difficult at best, and impractical in many cases. Data is frequently better transmitted in digital form, particularly where the monitor may be remote from the patient as, for example, when the data is to be transmitted by telephone connection to a physician at a different location.